Liquid crystal displays (LCDs) are widely used flat panel display devices. As is well known to those having skill in the art, a liquid crystal display generally includes a pair of spaced apart substrates with liquid crystals therebetween. Arrays of spaced apart data lines and gate lines define an array of pixels. A thin film transistor (TFT) for each pixel is electrically connected to a data line, a gate line and a pixel electrode.
LCDs may be classified by the orientation of the liquid crystals between the spaced apart substrates. In a twisted nematic (TN) LCD, the long axes of liquid crystal molecules are aligned parallel to the surfaces of the substrate, and the liquid crystal molecules are twisted between the two substrates. In contrast, in an in-plane switching (IPS) LCD, the liquid crystal molecules are rearranged to be parallel to the substrate upon application of an electric field therebetween.
LCDs may also be classified based on the manufacturing method that is used to form the channel region of the thin film transistors. In an etch stop-type LCD, an insulation layer is formed on a channel region of an amorphous silicon layer. Thus, the etch stop LCD may have a stable current characteristic, but the number of masks that are used may be increased. In contrast, in an etch back-type LCD, the amorphous silicon layer and a doped amorphous silicon layer are successively deposited and etched. Thus, the manufacturing process may be simple, but the amorphous silicon layer may need to be thick to obtain a channel region having a predetermined thickness, since part of the amorphous silicon layer may be removed when etching the doped amorphous silicon layer.
A conventional method of manufacturing an etch back-type thin film transistor LCD will now be described. In general, a plurality of spaced apart gate lines and an array of gate electrodes electrically connected thereto are formed on a substrate. A gate insulating layer is then deposited on the gate electrodes and the gate lines. An undoped amorphous silicon layer and a doped amorphous silicon layer are then deposited and etched to form active thin film transistor patterns. During these steps, organic impurities and an oxide film of nonuniform thickness may be formed on the doped amorphous silicon layer. The organic impurities and the oxide film may be removed by an aqueous hydrogen fluoride etch in a washing process.
A metal film is then deposited and patterned for form an array of spaced apart source and drain electrodes. A silicide layer may be formed to improve the contact characteristics between the doped amorphous silicon layer and the source/drain electrodes.
The doped amorphous silicon layer is then removed using the source/drain electrodes as a mask, to thereby form channel regions for the thin film transistors. During this removal, the amorphous silicon layer and the doped amorphous silicon layer may be nonuniformly etched, since the etch ratio of the silicide is generally different from the etch ratio of the amorphous silicon layer and the doped amorphous silicon layer. The nonuniform etching may degrade image quality of the LCD, since the leakage current may increase when the temperature of the substrate rises if the channel thickness is nonuniform. The degradation in image quality may be particularly severe with large and/or high quality LCDs.